1. Field Of The Invention
This invention relates to a method for the regeneration of anion exchange resins. More particularly, it relates to chemical displacement techniques using novel extraction mixtures and orders of addition to regenerate strong-base anion exchange resins. The technique is particularly suitable for the regeneration of anion exchange resins which have been synthesized to have a high specificity for large anions having low hydration energy.
2. Background Of The Art
Groundwater remediation has become an important issue in industrialized countries and particularly in those areas which draw drinking water from aquifers. In recent years, two contaminants have been identified which have been found to be difficult to remove when present in small concentrations. Perchlorates and pertechnetates are stable compounds which are highly mobile in underground aquifers. The salts are highly soluble and poorly retained by clays and other subsurface materials. The anions are not volatile, non-filterable, and are not removed by conventional methods such as sedimentation or air stripping techniques.
Perchlorates are manufactured and widely used during the twentieth century with particular application to explosives and rocket fuels. Their distribution is widespread, as documented by Damian, Environmental Protection, Jun. 24, 1999, and Urbansky, Bioremediation Journal 2, 81 (1998). Methods for quantitation of perchlorates have been developed only recently and chronic exposure data have not been definitively related to specific illnesses.
The pertechnetate anion is the primary chemical form of technetium-99, a (beta-emitting) radionuclide, with a half life of 2.13xc3x97105 years in oxygenated groundwater or surface water. Pertechnetate anions in water result almost exclusively from the fission of uranium-235 and plutonium-239. Except for fallout from above ground weapons tests, its occurrence in groundwater is believed to be limited to locations where fissionable material has been handled. Pertechnetate is chemically similar to perchlorate and subject to equivalent difficulties in treatment. Its radioactivity renders it more easily detected and more hazardous biologically.
The use of anion exchange resins in the concentration of pertechnetate is not new. Tc-99m (txc2xd= 6.0 h) generators for use in nuclear medicine routinely concentrate pertechnetate for patient administration. The concentrations are orders of magnitude higher than those in environmental contamination and the isotope is so short-lived that environmental contamination is not a concern outside of the medical facility. Representative ion exchange materials for this use include those disclosed in U.S. Pat. No. 4,738,834.
Anion exchange resins have been viewed by many as a preferred method for the removal of perchlorates and pertechnetates and various resins have been studied for removal of pertechnetates and perchlorates. Kawaski et al., Radiochimica Act 63, 53 (1993) reported adsorption on DOWEX(trademark) 1-X8 and suggested that perchlorate could be used to remove pertechnetate from the resin. Katty, U.S. Pat. No. 5,478,474 discloses a method for removing pertechnetate using phosphinimines covalently bound to a polymer backbone. Regeneration is not addressed in the paper.
Gu et al., Separations Technology 6, 123 (1996) addresses the adsorption and recovery of pertechnetate on activated carbon and desorption with anions such as phthalate, chloride, nitrate, sulfate, and salicylate. However, sorption on anion exchange resins and regeneration were not addressed in this paper.
Batista et al. (Paper ENVR 27) and Tripp et al. (Paper ENVR 24) disclose experiments in the regeneration of various anion exchange resins using sodium chloride and report that large volumes appear to be necessary to regenerate activity in resins which show some specificity toward perchlorate [Extended Abstracts, 218th ACS National Meeting, New Orleans 1999]. At the same meeting, Guter (Paper ENVR 23) published theoretical studies on the formation of ion pairs and calculated the order of affinity of singly charged ions on resins. Wachsmuth, U.S. Pat. No. 3,989,624, discloses a method for regenerating anion-exchange resins by backwashing with alkali metal hydroxide solution. Similarly, Salem et al., U.S. Pat. No. RE29,680, discloses a method for converting anion-exchange resins from monovalent anion to the hydroxide form. Fleming, U.S. Pat. No. 4,608,176, discloses a method of regenerating strong-base anion-exchange resins sorbed with thiocyanate using ferric ions in which ferric-thiocyanate complex cations are formed and washed from the resin. Whereas this last method uses a ferric salt, the regeneration method is substantially distinct from the method described in this invention and is ineffective in regenerating the anion exchange resins sorbed with perchlorate or pertechnetate because ferric ions do not form complex cations with perchlorate or pertechnetate. Fleming""s regeneration method consists of passing a solution of ferric sulfate, ferric chloride, or ferric nitrate in water through the resin, whereby the ferric ion forms a complex cation with the sorbed thiocyanate. The ferric-thiocyanate complex cations thus formed elute from the resin due to charge-repulsion, with the counter-anions (sulfate, nitrate, or chloride in ferric salt) replacing the thiocyanate as the anion on the resin. Thus, ferric cations are used to complex and desorb thiocyanate anions; the thiocyanate anions are not displaced by a secondary anion. As stated above, this method does not work with resins sorbed with perchlorate or pertechnetate and differs from the present invention, whereby ferric chloride is dissolved in a solution (containing HCl, a water-miscible organic solvent, and optionally an alkali-metal chloride salt) to generate some amount of the complex anion tetrachloroferrate, and this solution is passed through the resin whereby the tetrachloroferrate displaces other anions already sorbed on the resin such as perchlorate or pertechnetate. The sorbed tetrachloroferrate is then desorbed from the resin using a second regenerant.
Brown et al, Perchlorate in the Environment, E. T. Urbansky, ed. (Kluwor Academic/Plenum) describe a new resin specific for perchlorate and pertechnetate.
Highly selective anion exchange resins offer some advantages over conventional nonselective resins in the treatment of perchlorate-and pertechnetate-contaminated water because of their relatively high efficiency and capacity. For example, Oak Ridge National Laboratory has recently developed a new class of bifunctional anion exchange resins, which are highly selective and efficient for the removal of perchlorate, pertechnetate, and perrhenate from contaminated water (U.S. patent application Ser. No. 08/922,198 U.S. Pat. No. 6,059,975 herein incorporated by reference). However, because these anions are so strongly sorbed, the conventional regeneration technique by washing with a brine (e.g., 12% sodium chloride) or alkali metal hydroxide [U.S. Pat. Nos. 4,151,079; 4,049,772] is ineffective, and no current technology is available to regenerate these highly selective resins so that they can be reused routinely. Because most of these synthetic resins are expensive with a current market price of xcx9c$400 to xcx9c$1000 per cubic foot, the resin itself contributes to a major capital cost for the application of ion-exchange technology to remove perchlorate or pertechnetate from contaminated water or other liquid streams.
The need exists, therefore, for a method whereby the resins specific for perchlorate and pertechnetate may be regenerated for multiple reuse in a manner which concentrates the removed perchlorate and pertechnetate and which can be performed at a minimal cost compared to the cost of resin replacement. The method should be suitable for both the treatment of large aquifers or bodies of standing water as well as for localized facilities to provide potable water on site.
It is an object of this invention to provide regenerants for anion exchange resins having a specificity for large, poorly hydrated anions including perchlorate, pertechnetate, nitrate, periodate, perrhenate, iodide, and thiocyanate. It is a further object of this invention to provide regenerants which may be reused on multiple occasions. It is a further objective of this invention to isolate perchlorate and pertechnetate in a concentrated form so that economic treatment (destruction for perchlorate or burial for pertechnetate) can be done with minimal expense.
These and other objectives of this invention may be achieved by the regeneration of the anion exchange resin using anions formed in newly formulated regenerant solutions having a selectivity toward the cationic site of the resin which is approximately equivalent to that of the perchlorate and the pertechnetate so that, under regeneration conditions, these anions displace perchlorate or pertechnetate. It is a further component of this invention that the regenerant used to displace the perchlorate or pertechnetate may then be efficiently displaced or decomposed with at least a second regenerant anion which may be retained on the resin or exchanged facilely with the anion present on the resin as initially activated.
In a first embodiment, the invention contemplates the use of tetrachloroferrate (FeCl4xe2x88x92) anion generated in situ, followed by a wash with dilute hydrochloric acid. Alternatively, tetrafluoroborate (BF4xe2x88x92) may be used to displace the perchlorate or pertechnetate and it is likewise displaced by an anion having a lower specificity and selectivity such as nitrate, hydroxide or salicylate. A final brine and/or a dilute hydrochloric acid wash is sufficient to activate the resin in the chloride form. As a third alternative, salicylate anion generated in a base may be employed as the displacing anion and the resin regenerated in the chloride form by washing with dilute hydrochloric acid. Organic solvents miscible in regenerant solutions may be used to adjust the ionic strength and polarity of the regenerant to maximize the efficiency of displacement.
The objects of the invention, therefore, provide a method and a process for regenerating anion exchange resins either in a fixed bed, in continuous rotating column apparatus, or in a batch process that are used for removing specific anions such as perchlorate, pertechnetate, nitrate, periodate, perrhenate, iodide, thiocyanate, and some organic anions from water or other liquid streams.